Without limiting the scope of the invention, its background is described in connection with imaging techniques used in oil recovery.
Accurate, non-invasive determination of oil saturation distribution in laboratory cores, near-wellbore zones, and deep in the reservoir, will greatly improve understanding of oil displacement mechanisms for various enhanced oil recovery (EOR) processes, and also help identify the location of bypassed oils so that they can be subsequently recovered. Currently, the oil saturation distribution can be determined: (i) for laboratory cores, using MRI or CT-scan imaging; (ii) for near-wellbore zones, by NMR and other logging methods; and (iii) for zones deeper in the reservoir, e.g., by injection of partitioning tracers. The difficulty with the NMR logging is that its probing depth is very shallow, i.e., in centimeters. Moreover, interpretation requires knowledge of rock surface properties (wettability, relaxivity) that can be difficult to estimate independently. With the “echo” injection/production of partitioning tracers, a reliable determination of oil saturation from the tracer material balance is difficult, because it is difficult to have the detailed knowledge of the porosity-permeability distribution in the study zone.
U.S. Pat. No. 4,769,602 issued to Vinegar and Tutunjian (1988) provides a method of imaging of materials to determine petrophysical properties and measuring fluid saturations using Nuclear Magnetic Resonance (NMR) Imaging. According to the '602 patent, an earthen core sample containing multiple fluids during a coreflood experiment is located within a NMR Imaging Apparatus, so that the sample and fluids therein may be imaged. In coreflood experiments conducted for reservoir engineering applications, the three fluid phases may typically be water or a brine phase, an oil or oleic phase, and a gas phase.
U.S. Pat. No. 7,170,294 issued to Kasevich (2007) discloses the application of electromagnetic tomography for efficient recovery of oil and gas as well as the removal of unwanted liquids from subsurface formations. The Kasevich patent involves the deployment of both surface and a single borehole magnetic dipole structures used for both transmitting and receiving low frequency electromagnetic energy. Several concentric surface antenna arrays with electronic switching between each circular array and the downhole solenoid will have the ability to image the spread and movement of oil and gas during thermal treatment and provide three dimensional temperature measurement. The oil movement to recovery wells may be provided by radio frequency heating or steam flood as in enhanced oil recovery. This information allows for developing very efficient oil and fluid recovery techniques by actually observing topographic images developed according to this invention.
U.S. Patent Application No. 20090167302 (Edwards and Ladva, 2009) discloses the use of time-lapsed NMR diffusivity measurements in an observation well. The observation well is cased in the zone of interest with non-magnetic and non-conductive casing that is invisible to the NMR tool. Secondly, because NMR measurements have a dead zone in front of the antenna depending on the spatial variation of the fixed magnet strength, for example about 2.7 inches, a distance between the casing and the formation is reduced to less than the dead zone length by drilling the observation well at small deviation of about 5° and running the casing without centralizers. Both the casing and the pad-type NMR tool will follow the low side of the borehole, ensuring the measurement volume of the NMR tool is inside the formation and beyond the annulus. With the appropriate observation well completion, time-lapse diffusivity measurements with pad-type NMR tools can address several shortcomings in the current practice of monitoring EOR processes that rely upon changes in density and hydrogen index (HI). Various uses of NMR imaging in wells cased with non-metallic casing are also disclosed.